1. Field of the Invention
The present invention relates to a video apparatus such as a color television camera and, more particularly, to an apparatus for coupling to an image splitting prism solid-state image sensors for respectively receiving component images decomposed by the image splitting prism.
2. Related Background Art
A relative positional relationship between a light exit surface of each color channel of an image splitting prism and a light-receiving surface of the corresponding solid-state image sensor must be determined in consideration of focus adjustment (optical axis) for each color channel, adjustment of inclination of the light-receiving surface to eliminate defocusing on one side of a screen, and adjustment of registration (parallel and rotational adjustment within a plane perpendicular to the optical axis) between the color channels. In a system wherein a board is mounted with an image splitting prism for allowing the above adjustment operations or each solid-state image sensor is mounted in a housing through an adjustable holding mechanism, the image pickup apparatus itself has a complicated holding mechanism. A system for directly adhering solid-state image sensors respectively to the light exit surfaces of the color channels of the image splitting prism by using a transparent adhesive is disclosed in Japanese Utility Model Laid-Open No. 54-111419. According to this system, although it is possible to perform adjustment of a position within a plane perpendicular to the optical axis and in a rotational direction, adjustment (focusing, and defocusing on one side of the screen) in the axial direction cannot be performed.
A system for achieving adjustment in the axial direction is disclosed in Japanese Patent Laid-Open No. 58-63279. According to this system, a flexible spacer is interposed between a light exit surface of an image splitting prism and an edge portion of a solid-state image sensor and an adhesive is applied to side surfaces of the spacer to maintain a flexibly deformed state upon hardening of an adhesive. When a linear expansion coefficient of an adhesive is large, a focusing error is not negligible when a solid-state image sensor having a large focusing error caused by a change in humidity and a high resolution is used. According to this system, focusing and registration errors are increased due to deterioration over time since a thick adhesive layer is formed. It is possible to adhere a metal mounting piece to an image splitting prism and hold the solid-state image sensor by this metal mounting piece. When an abrupt change in temperature occurs, however, a large difference between the linear expansion coefficient of the material of the image splitting prism and that of the mounting piece causes a large force to act on the prism, thereby damaging the prism. It is also difficult to remove dust when it is attached to a corner at which the mounting piece is adhered to the prism surface.
When one of the solid-state image sensors has failed, the failed element must be removed from the image splitting prism and replaced with a new one. When the failed element is removed from the prism, the surface of the image splitting prism may often be damaged.
In Japanese Utility Model Laid-Open No. 59-39580, a column is mounted on an exit surface of an image splitting prism, and the column is soldered to a mounting piece for holding a solid-state image sensor as in bonding of electrical components. Bonding by soldering is suitable for removal of a solid-state image sensor.
In Japanese Patent Laid-Open No 61-135279, support members each having an opening are respectively adhered to exit surfaces of a color separation prism. A plurality of legs of this support member are soldered to legs of the mounting piece.
During assembly, however, it is difficult to remove small dust from a portion where an edge of the opening of the support member contacts the prism. Residual dust causes scattering of light during an image pickup operation, thereby degrading image quality. Other various problems caused by adhesion of metallic members to the prism using an adhesive are also posed.
Other conventional devices in which solid-state image sensors are coupled to image splitting prism are known in U.S. Pat. Nos. 4,268,119, 4,323,918, 4,591,901, and 4,623,916.
The assignee of the present applicant has proposed Japanese Utility Model Laid-Open No. 63-81481 (U.S. Ser. No. 119,093) and Japanese Utility Model Laid-Open No. 1-23186. In these proposals, in order to overcome the above-noted problems, a coating layer consisting of a special-purpose solder is formed on a glass block, and a normal solder is used to fill a gap between the coating layer and a holding tool for holding the solid-state image sensor, thereby bonding the glass block and the holding tool. When a product is manufactured by this method, however, the thickness of the solder layer is excessively large so as to degrade its strength. In order to prevent this, the special-purpose solder layer is cut to have a very small thickness and is then soldered to the holding tool. In this case, the thickness of the solder layer requires high precision, and the soldering operation is cumbersome. In addition, when the solid-state image sensor is repeatedly adhered to or removed from the holding tool through the solder layer, and particularly when the special-purpose solder layer is repeatedly heated, the adhesion strength between the glass block and the solder layer is often weakened. Skills are required to apply a special-purpose solder to glass to obtain a desired shape while an ultrasonic wave is being applied to the solder. A device having a solder layer directly formed on a glass block is disclosed in U.S. Pat. No. 4,659,378.
Japanese Utility Model Laid-Open No. 63-81481 discloses a metallization method in addition to the method of forming a coating layer as a special-purpose solder layer.